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Respiratory system

(apparatus respiratorius)

The principal function of the respiratory organs is the Conducting airways
uptake of oxygen from the atmosphere to sustain aerobic
cellular metabolism, and the expulsion of carbon dioxide Structure of the conducting airways
produced during cellular respiration. The vascular system Most of the internal surface of the conducting airways is
serves as a vehicle for transport of gases between the lined by respiratory mucosa. A small portion, restricted
respiratory system and the cells of the body. Respiratory to the olfactory region of the nasal cavity, is classified as
activity influences the concentration of oxygen and carbon olfactory mucosa.
dioxide in the blood, thus regulating blood pH. The structure of the respiratory mucosa is consistent
The respiratory system is divided into two structurally throughout the conducting airways (Figure 11.1 and Table
and functionally distinct components: 11.1), comprising:

· conducting airways for transport of inspired and · ciliated pseudostratified columnar epithelium (respi-
expired gases and ratory epithelium) underlaid by
· an interface for passive exchange of gases between · a fibro-elastic network, mixed glands, smooth muscle
the air and the blood (blood–air barrier). cells, and numerous aggregates of lymphatic tissue and
· occasional capacitance vessels (veins), particularly in
The conducting airways begin at the nasal cavity and con- the nasal cavity.
tinue through the pharynx, larynx and the trachea to the
bronchi within the lungs. Throughout its course, inspired Except for the nasal vestibule, pharynx and larynx, the con-
air is filtered, humidified, warmed or cooled and subjected ducting airways are lined from the nasal cavity to the distal
to sensory sampling by the olfactory region (fundus nasi). bronchi by respiratory epithelium (Figure 11.2). This
The respiratory mucosa performs innate and acquired typically pseudostratified columnar epithelium contains
immune functions. various cell types:
Organs lying adjacent to the upper respiratory tract,
including the paranasal sinuses, auditory tube, vomerona- · ciliated epithelial cells,
sal organ, nasolacrimal ducts and the guttural pouches (in · secretory cells (mainly goblet cells),
the horse), perform ancillary functions that complement · brush cells with microvilli,
the role of the conducting airways. The larynx is special- · basal cells (acting as reserve cells) and
ised for vocalisation. · APUD cells.
The conducting airways end in a series of increasingly
narrow tubes that open into blind sacs referred to as alve- The free surface of most epithelial cells bears cilia that beat
oli. These terminal segments constitute the respiratory in a coordinated fashion to transport inhaled foreign mat-
system in the strictest sense, as this is where gas exchange ter, or particles originating from within the body, towards
occurs. The alveoli are thin-walled chambers surrounded the pharynx. To facilitate this function, particles are bound
by a dense network of capillaries. The blood-air barrier is to cilia by mucus produced by numerous secretory cells
formed by the epithelium of the alveoli and the endothe- within the epithelium. Secretory cells (predominantly gob-
lium of the adjacent blood vessels with its underlying basal let cells) are found mainly in the upper airways, preventing
lamina. build-up of mucus in the lungs. Together with subepithe-
The pulmonary capillaries also play a role in regulation lial mixed glands, the mucus produced by goblet cells also
of the circulatory system. The endothelium synthesises serves to humidify inspired air.
serotonin, histamine and bradykinin, and converts angio- Brush cells, featuring prominent microvilli, are specialised
tensin I into angiotensin II. chemosensory cells. Basal cells serve as reserve cells, capable

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